This invention relates to the art of blasting and, more particularly, to a method and means for securing signal transmission from a detonating cord and to one or several shock wave initiable low energy fuses.
For the purpose of transmitting initiation signals to explosive charges, various types of fuses are used as alternatives to electrical means. For reproducible timing between different charges it is necessary to use fuses of high signal speed in which a shock wave is rate determinant for transmission. High speed fuses are of two general types. Detonating cords with a relatively high loading of core explosives are structurally destroyed when used but are able to initiate nearby explosives. Low energy fuses have a lower core loading resulting in a weak shock unable to initiate nearby explosives and often leaving the external fuse layers structurally unchanged after use.
It is often desirable to combine the two types of fuses in the same blasting network. For example, low energy fuses are preferred for the branches leading down the boreholes since they allow bottom initiation with a blasting cap without initiation, compression or other negative effects on the charge from the descending impulse. Detonating cords have some practical and economical advantages when used as surface trunk lines for connection and common initiation of the branches, especially when the number of branches is large.
The connection between a detonating cord and a low energy fuse is a critical point on which several requirements must be placed. For ignition safety reasons the cord is often laid out in loops which, however, makes it difficult to foresee from which cord direction the detonation will reach a particular branch. Hence the connection shall be bi-directional and independent of signal arrival direction. The point of contact between cord and fuse must be sufficient for ignition of the latter considering the short duration of the shock wave passage. If improperly designed, even an extended contact line may result in ignition failures since the cord impulse is generally faster than the fuse impulse, so that it may overtake an already established ignition in the fuse and extinguish it or cut the fuse in front of the impulse. The same results may be caused by splinters from ill-designed blocks and fixtures. The cord may also cut itself if folded so that a portion in advance of the detonation front is reached by a shock sufficient for tearing but insufficient for ignition. Finally it is desirable that the connection can be made easily and with reproducible results without too high requirements for skill.
Hitherto used connection means have only partially fulfilled the desired requirements and pick-up failures have not been uncommon. The simplest method of making the connection with a hand-tied knot does not give reproducible results unless substantial skill is exercised. Only certain types of knots re reliable and even with these a too rigid knot may compress and inhibit the fuses while a too loose knot may give tearing problems. The fuse and cord parts immediately surrounding the knot often become randomly oriented. Knots are seldom symmetrical and fully bi-directional. Various hooks and clips are also in use, either very simple in design or more elaborate like the devices shown in U.S. Pat. Nos. 3,175,491 or 3,431,849. In general these devices indeed give more reproducible positioning of the fuse parts but instead provide a too limited contact area and neglects the need for sustained suspect of the initial ignition.